def isPrime(a):    
    if a == 1: return False    
    test = True    
    k = a - 1    
    while k > 1:    
        if not a % k:    
            test = False                
            break    
        k -= 1        
    return test   
      
n = input()  
r = []  
for i in range(len(n)):  
    for k in range(i + 1,len(n) + 1):  
        a = int(n[i:k]) 
        if isPrime(a):  
            r += [a]  
print(max(r) if r else 0)